(i) Field of the Invention
The present invention relates to a method and to a device for creating a uniform discharge at atmospheric pressure (non-filamentary) in a gas, in particular in nitrogen or in a gas mixture based on nitrogen, and to its application in particular to surface treatment using a plasma at atmospheric pressure or alternatively to the breakdown of effluents.
(ii) Description of Related Art
One of the applications relates more particularly to treatments for modifying the surface characteristics of a polymer film with a view, for example, to modifying its wettability or forming chemical bonds capable of improving the adhesion of a future coating.
In methods for surface treatment using a plasma, the surface to be treated is traditionally brought into contact with a plasma created by ionizing a treatment gas so as to create excited chemical species and electrons, which in particular can collide with third species (in particular neutral ones).
These collisions can cause energy to be transferred to the species so as to form new chemically active species, in particular so-called "metastable" species, atomic species, radicals, ions and electrons. Chemically active species with a long lifetime can in turn collide with third species, which can thus create other active species and electrons.
The active species can interact with the surface of an article in contact with the gas, which makes it possible to modify the surface characteristics of this article, on the one hand creating new molecular structures capable of interacting with the molecules of a coating deposited later, and on the other hand modifying the morphology of the material and, in particular, the mobility of chains or chain segments in the case of polymers.
It is known that electric discharges at atmospheric pressure have the advantages, on the one hand, of not requiring the use of bulky and expensive devices for creating a powerful vacuum in the treatment reactor in which the discharge is formed and, on the other hand, of permitting continuous treatments which are compatible with productivity requirements.
The electric discharges employed at atmospheric pressure, such as "corona" discharges, are generally referred to as "filamentary" since they are generated in the form of sorts of current microchannels, typically some hundred microns in diameter, which develop randomly in space and time between the two electrodes.
However, it has been found that such a "filamentary" discharge technique is not efficient enough at the microscopic level because it has a number of drawbacks at the microscopic level. Specifically, the discharge created in this way is microscopically nonuniform since, firstly, between two filaments, the surface has untreated regions and furthermore, even at the individual filaments, the treated surface is liable to be degraded by an excessive number of polymer chain breaks or even local heating.
Attempts have been made to overcome this drawback by subjecting the surface to be treated to a uniform discharge, that is to say a discharge in which the excited chemical species are regularly distributed over the surface to be treated, while of course maintaining atmospheric pressure conditions (specifically, attempts could have been made to make the discharge uniform by reducing the pressure).
It is thus known to produce a uniform discharge at atmospheric pressure in a neutral gas, essentially helium (reference may for example be made to documents U.S. Pat. No. 5,456,972 and EP-A-346 055). However, for treating surfaces with such a plasma created in a gas based on nitrogen, at least one electrode in the form of a grill is still used.
However, work carried out by the Applicant Company has shown that such electrodes do not make it possible for the uniformity of the discharge created to be improved satisfactorily at the microscopic level.